1. Field of the Invention
This invention relates to a spray drying process for recovering polymer from its solution and more particularly, to a process for separating a polymer from solution by flashing the solution in a heated atmosphere.
2. Description of the Prior Art
In many methods for preparing normally solid thermoplastic polymers, solvents which are inert and liquid under contacting conditions are employed to facilitate the polymerization reaction and aid in subsequent purification steps such as monomer recovery and catalyst removal. In such processes, the solvent must be removed with any other vaporizable material in order to recover the solid polymer in a usable form. Residual amounts (often as low as 1%) of light components in the polymer will vaporize upon heating, as in injection molding, thus forming both bubbles and cavities in the finished product. For many applications of such polymeric material, the allowable limits of vaporizable components is less than about 1% and often is less than 0.1%.
The separation of solvent from the polymer presents several problems, particularly when the polymer concentration is initially low, for example, 10 weight percent or less. Vaporization processes are hampered because the viscous nature of the polymer solution reduces heat transfer and can cause considerable foaming as the polymer concentration increases. One method of producing a polymer with less than 1 weight percent residual solvent is by flashing the polymer solution under such conditions that a substantial portion of the solvent is vaporized and a solid polymer is precipitated; however, quite often with commercially available spray drying equipment, this solid polymer is a highly expanded cellular material having a bulk density which may be as low as one pound per cubic foot. This light material is difficult to handle in subsequent operations and it cannot readily be fed to extrusion apparatus at a satisfactory rate because of its low bulk density. Remelting this material is extremely difficult because its insulating effect is such that decomposition temperatures may be reached in some regions before other parts of the material become soft.
A typical flashing procedure is disclosed in U.S. Pat. No. 2,964,513. The process of this patent is particularly adapted to the recovery of polyolefins. According to the invention of this patent, a solid polymer product is formed by dispersing a relatively dilute polymer solution in a flashing zone wherein contact is made with a hot inert gas so that the polymer is heated above its melting point but below the temperature at which the polymer would become fluid. Gases which may be used in the process include nitrogen, carbon dioxide, stack gases, vapors of hydrocarbon, and steam. According to the process, polymer solution is fed to a flash chamber through a nozzle which disperses the solution into droplets. The dispersed solution is immediately contacted with the hot inert gas which strips solvent and light waxes from the solution and causes the polymer to precipitate as a solid. Vapors are collected and condensed in a condenser and subsequently purified to remove the low molecular weight polymer and waxes. Precipitated polymer in contact with the hot gas is heated to a temperature above its melting point and densifies it as it continues to fall to the bottom of the flash chamber. As the polymer particles fall into the lower section of the chamber, they are contacted with low temperature steam and cooled to a temperature below their softening point. The densified polymer is then processed in an extruder wherein substantially all of the residual solvent is said to be removed. This procedure is not applicable to polymers having high melting points such as the aryl substituted polyphenylene ethers.
Another procedure for recovering polymer from solution is disclosed in my U.S. Pat. No. 3,241,600. In this patent, a process is disclosed including pre-heating of a polymer solution (especially that of an elastomeric polymer) to a temperature above the boiling point of the solvent while maintaining it under sufficient pressure to keep the solvent in a liquid condition. The solution, thus superheated and pressurized, is passed through an atomizing nozzle from which the solution emerges as an extremely finely divided spray into a chamber maintained at a lower pressure than the superheated solution, typically maintained at substantially atmospheric pressure, with the result that the bulk of the solvent immediately flashes off, leaving the particles of solid polymer behind. The process is carried out above the surface of water maintained at an elevated temperature. The particles, still containing some solvent, fall into the water and a certain amount of agglomeration of the particles takes place. The particles are then recovered from the water by a process such as filtration.